The invention relates in particular to a method and apparatus for treating glass bodies that are equipped with measuring components and are used for chemical sensors, in particular pH electrodes of the kind described, e.g., in the following product sheets of Mettler-Toledo GmbH, CH-8902 Urdorf, Switzerland: “Low-maintenance pH electrodes and systems”, September 2002; “InPro 2000 pH-Elektroden mit Flüssigelektrolyt und integriertem Temperaturfühler” (InPro 2000 pH electrodes with fluid electrolyte and integrated temperature sensor), October 2000; or “InPro 3200 (SG) pH-Elektroden mit Gelelektrolyt und integriertem Temperaturfühler” (InPro 2000 pH electrodes with gel electrolyte and integrated temperature sensor), January 2002.
The principal arrangement of the pH electrodes, which are configured as combination electrodes and include a glass electrode and a reference electrode, is presented in the above-cited references, as well as in the attached FIG. 1. Under the design concept of the combination electrode, the glass electrode which has a lead-off element 281 and the reference electrode which has a reference element 282 are built as a combined unit. The reference electrode surrounds the glass electrode like a ring. The functional principle of these pH electrodes is described in Charles E. Mortimer, “Chemie, Das Basiswissen der Chemie”, 5. Auflage, Georg Thieme Verlag, New York 1987, pages 337-338, with reference to Figure 20.9 which illustrates an experimental setup.
Inside a first chamber 291, which is enclosed in an interior tube 21 and terminates in a thin-walled glass hemisphere or glass membrane 25, the lead-off element 281, which normally consists of silver/silver chloride, is immersed in a solution of defined pH value or in an inner buffer 271 representing the electrically conductive connection between the inside surface of the glass membrane 25 and the lead-off element 281 (concerning buffer see Mortimer, page 292).
The semi-permeable glass membrane 25 is pH-sensitive. An electrical potential, which occurs across the semi-permeable glass membrane 25 represents a direct measure for the pH value of the tested solution or other substance to be measured. As soon as the pH electrode is dipped into the substance to be measured, the glass membrane 25 begins to swell on the outward-facing side, as sodium ions Na+ are replaced by hydrogen ions H+. The inside is always in a swollen state as it is permanently wetted by the inner buffer 271. The pH value of the inner buffer is normally set at pH7, i.e., at the neutral level. The swollen surface layers, which have a depth of less than 0.0001 mm, can absorb the hydrogen ions of the solution and the ions of the inner buffer by diffusion. If the pH electrode is immersed in a test substance with the same concentration of protons as the inner buffer 271, the difference between the respective electric charges of the inner buffer and the test substance is ideally equal to zero. Consequently, no electrical potential occurs in this case across the glass membrane 25. From the absence of an electrical potential, one can derive the conclusion that the test substance has likewise a pH value of 7. If the test substance has more or fewer positive charges than the inner buffer 271, there will be a difference in the electrical potential, where the polarity of the difference indicates whether the test substance has a surplus or deficit of positive charges.
The voltage potential that occurs in the lead-off element 281 is compared to the voltage potential that establishes itself at the reference element 282. Under idealized assumptions, the voltage potential at the reference element 282 remains constant, independent of the ion concentration in the test substance. The difference between the two voltage potentials forms the actual measuring signal which provides information about the ion concentration in the test substance.
The reference element is immersed in an electrolyte, normally a potassium chloride (KCl) solution 272, and conductively connected to the latter through ion migration. The KCl solution 272, which is enclosed inside a second chamber 292 between the outside wall of the interior tube 21 and the inside wall of the exterior tube 22 diffuses slowly through a porous separating wall or diaphragm 26 into the test substance and thereby establishes the electrical connection to the latter. It is important for the diaphragm 26 to be fluid-permeable for the KCl solution 272, but on the other hand, the test substance cannot be allowed to migrate from the outside into the KCl solution 272. This can be prevented for example by always keeping the top surface of the KCl solution 272 at a higher level than the top surface of the test substance. Furthermore, the outward diffusion of the KCl solution should be as strong as possible in order to keep the internal electrical resistance small. Thus, the diaphragm 26 is a porous separator between the KCl solution 272 and the test substance, which normally has a different ion concentration. The diaphragm 26 prevents the solutions on the one hand from equalizing their levels of ion concentration, while on the other hand an ion stream flows through the diaphragm.
The glass membrane 25 consists of a special glass with a thickness of for example 0.3 to 0.5 mm, which is preferably blown into a hemispherical shape to optimize its mechanical stability. The composition of the glass is for example 72% SiO2, 22% Na2O, and 6% CaO, which can be obtained by melting corresponding quantities of SiO2, Na2CO3 and CaCO3.
The preferred way of producing the glass body of the pH electrode is to use an immersion tube 2 which, as shown in FIG. 2a, has an exterior tube 22 whose inside wall is connected to the interior tube 21 by means of a ring plate 23 so as to form a first chamber 291 which is to be closed off on one side by the glass membrane 25, and a second chamber 292 which is closed off on one side by the ring plate 23.
To form the glass membrane 25, the immersion tube 2 is introduced into a crucible containing molten glass and a small quantity of the molten glass, a so-called gob 24 is taken out which attaches itself to the lower rim of the exterior tube 22, as shown in FIG. 2b. Through an in-flow of a gaseous medium, the molten glass gob 24 can be blown into a thin-walled hemispherical glass membrane 25, whereby the glass body 20 shown in FIG. 2c is produced.
FIG. 3a shows the glass body 20 of FIG. 2c with the first chamber 291 filled with an inner buffer 271 and with a lead-off element 281 installed. The lead-off element 281 has a terminal wire 285 which is connected, preferably by means of a press-clamped connection 284, to a plug contact 283 which serves to connect the measuring instrument. Of course, there are other ways to connect the terminal wire 285 to the measuring instrument. For example, a connecting portion of the terminal wire 285 can be taken to the outside of the glass body 20 and electrically contacted, e.g., by soldering.
To separate the first chamber 291 and the second chamber 292 from each other, it is preferred to seal the first chamber 291 tightly after the chamber 291 has been filled with the inner buffer 271 and the lead-off element 281 has been installed. Among other things, this serves to avoid contamination of the inner buffer 271, for example when contacting the plug connector 283.
Under the prior art concept, which is illustrated in FIG. 3b, a part 220 of the exterior tube 22 was removed in order to expose the interior tube 21, so that a portion 210 of the interior tube could be heated by means of a burner and fused together. Thus, the first chamber 291 is closed off by the fused portion 210 of the interior tube 21, whereby the inner buffer 271 is prevented from leaking out, and foreign substances are prevented from entering.
After the interior tube has been fused, the separated portions of the exterior tube 22 need to be joined again. The rejoined portions need to be in precise coaxial alignment so that, e.g., a pH electrode equipped with the finished glass body 20 can be installed with a precise fit in an appropriate receptacle, for example in an armature.
However, the foregoing prior-art method of producing glass bodies and, more specifically, of fusing an interior tube of a glass body containing measuring components, can only be performed by experienced personnel at considerable expense.
A method and apparatus for producing a fluid electrode with an interior tube and an exterior tube are described in U.S. Pat. No. 4,661,236 to Gelo (“Gelo '236”), wherein the interior tube is filled with a solution in which a metal contact is immersed. The metal contact passes from the interior tube through a hermetically sealed closure which separates the fluid from the surrounding space. The method is distinguished by the fact that the interior tube consists of a glass that absorbs electromagnetic waves of selected wavelengths, while the exterior tube is transparent for the same wavelengths with almost no absorption. To produce the hermetic closure, the interior tube is fused shut through irradiation at an appropriately selected wavelength, so that the tube melts and contracts itself around the metal contact.
The foregoing method can pose a problem that occurs in a process step prior to the fusing of the interior tube, specifically in the step of forming the membrane, in that the heat which is radiated from the molten glass in the crucible is absorbed by the material of the interior tube, whereby the connection between the interior tube and the exterior tube, e.g., by way of the aforementioned ring plate, can come loose. According to Gelo '236, this problem can be solved by using two different types of glass for the interior tube, where the lower portion of the interior tube is made of the same type of glass as the exterior tube, and the upper portion of the interior tube is made of the type of glass that absorbs the electromagnetic waves.
This kind of tube, which is made of two types of glass, is relatively expensive to produce. Moreover there is a risk, when the tube contracts in the melting region, in that the closure between the wall of the tube and the metal contact will occur not to be complete. Therefore the described method of fusing needs a length of time that is exactly scheduled. Furthermore, controlling the exact position of the fusion of the interior tube requires a complex optical adjustment in order to focus the radiation on the right spot.
U.S. Pat. No. 3,855,095 to Leonard discloses an electrode having an interior tube and an exterior tube both made of glass, wherein the interior glass tube is fuse-melted by means of a glass element. The fuse-melting process occurs by means of inductive heating, wherein a tool is used for the coupling of the heat to the glass element. The application of means to produce sonic energy is proposed as well.
The present invention therefore has the objective to provide an improved process and an improved apparatus for treating multi-walled glass bodies, further to provide a glass body that is produced in accordance with the improved method, as well as a measuring probe that is equipped with the glass body.
In particular, the invention aims to provide an improved method of treating multi-walled glass bodies, wherein the method offers an inexpensive way in which chambers inside a glass body can be tightly closed off.
As a further objective, the invention aims to provide an apparatus that is inexpensive to build and easy to operate, whereby glass bodies can be finished rapidly and precisely, preferably through an automated process.